I'm a bit worry about that slapd will use all of the memory
and kernel kills it violently (=> database corruption etc)
It seems slapd is reserving 1.6G of memory.
ldif dump of whole database is 84 megabytes of disk.
At this time there is 340 TCP connections to server.
Is this normal for slapd-2.1.30?
If it is we just have to buy more RAM :)

I found OL 2.1.30 slapd running on RHEL 3 to have a memory leak
that was exercised by searches against the directory. We had to
restart slapd every couple of days to prevent it from eventually
consuming all memory on the host. The leak is not present (but a
lot of wonderful features are) since OL 2.2.23 slapd, so you might
try upgrading to the latest stable version.

For backend we use bdb without DB_CONFIG (for defaults)

You *definitely* need a DB_CONFIG tuned to your particular
circumstances. Don't rely on the default settings, which are
inadequate for almost every reasonable OpenLDAP use case. There is
a wealth of good information about proper DB_CONFIG tuning in the
OpenLDAP FAQ and in the list archives. Be sure to follow the links
from the FAQ articles to the SleepyCat BDB docs and read those too.

slapd.conf
...
threads 25

I recommend that you leave your threads setting at the default until
you've properly tuned a DB_CONFIG file, then experiment carefully
(if needed) to get the best setting.